The use of portable electronic instruments is increasing as electronic equipment gets smaller and lighter due to developments in the high-tech electronic industry. Studies on lithium secondary batteries are actively being pursued in accordance with the increased need for a battery having high energy density for use as a power source in these portable electronic instruments. Lithium-containing metal oxides are used as a positive active material of a lithium secondary battery, and lithium metals, lithium-containing alloys, crystalline and amorphous carbons, and carbon-containing composites are used as a negative active material of a lithium secondary battery.
The average discharge voltage of a lithium secondary battery is about 3.6 to 3.7 V, which is higher than other alkali batteries, Ni—MH batteries, Ni—Cd batteries, etc. However, an electrolyte that is electrochemically stable in the charge and discharge voltage range of 0 to 4.2 V is required in order to generate such a high driving voltage. As a result, a mixture of non-aqueous carbonate-based solvents, such as ethylene carbonate, dimethyl carbonate, diethyl carbonate, etc., and a lithium salt, such as LiPF6, LiBF4, or LiClO4, is used as an electrolyte. However, the ion conductivity of such an electrolyte is significantly lower than that of an aqueous electrolyte that is used in a Ni—MH battery or a Ni—Cd battery, thereby resulting in the deterioration of battery cell performance during charge and discharge at a high rate.
Charge and discharge characteristics are affected by ionic conductivity of an electrolyte, and it is preferable that the ionic conductivity is high. Since a large amount of free ions are capable of increasing the ionic conductivity (the ionic conductivity results from a large amount of free ions), the dielectric constant of the electrolyte is high and the viscosity electrolyte of the electrolyte is low. In addition, the electrolyte has a low freezing point, resulting in good movement of the free ions. (Makoto Ue, Solution Chemistry of Organic Electrolytes, Progress in Battery Materials, Vol. 16 (1997).)
U.S. Pat. Nos. 5,639,575 and 5,525,443 disclose a solvent having a high dielectric constant mixed with a solvent having a low viscosity in order to enhance electrochemical characteristics of lithium ion batteries, and in particular, a solvent having a low freezing point mixed therewith in order to enhance electrochemical characteristics of lithium ion batteries at low temperatures. However, when a lithium ion battery is discharged at a high rate (1C), its discharge characteristics deteriorate rapidly because the lithium ion mobility is so low at low temperatures, especially at −20° C. Therefore, in order to enhance its discharge characteristics at a high rate, an electrolyte must have high ionic conductivity and low internal resistance when a lithium ion battery is discharged at a high rate.
During the initial charge of a lithium secondary battery, lithium ions, which are released from the lithium-containing metal oxide positive electrode of the battery, are transferred to a carbon negative electrode where the ions are intercalated into the carbon. Because of its high reactivity, lithium is reacted with the carbon negative electrode to produce Li2CO3, LiO, LiOH, etc., thereby forming a thin film on the surface of the negative electrode. This film is referred to as a solid electrolyte interface (SEI) film. The SEI film formed during the initial charge not only prevents the reaction between lithium ions and the carbon negative electrode or other materials during charging and discharging, but also acts as an ion tunnel, allowing the passage of only lithium ions. The ion tunnel prevents the disintegration of the structure of the carbon negative electrode, which causes organic solvents in an electrolyte with a high molecular weight to make solvate lithium ions, and the solvent and the solvated lithium ions co-intercalate into the carbon negative electrode.
Once the SEI film is formed, lithium ions do not again react with the carbon electrode or other materials such that an amount of lithium ions is maintained. That is, carbon from the negative electrode reacts with an electrolyte during the initial charging, thus forming a passivation layer, such as an SEI film on the surface of the negative electrode, such that the electrolyte solution no longer decomposes and stable charging and discharging are maintained (J. Power Sources, 51(1994), 79-104). As a result, in the lithium secondary battery there is no irreversible formation reaction of the passivation layer, and a stable cycle life after the initial charging reaction is maintained.
Since the characteristics of the SEI film are affected by the kind of solvent used for an electrolyte and additives, and they affect ion movement and charge transfer, the battery efficiency may be changed by them, and they are critical for the battery efficiency. (Shoichiro Mori, Chemical Properties of Various Organic Electrolytes for Lithium Rechargeable Batteries, J. Power Sources, 68 (1997).)
In order to enhance the characteristics of the SEI film, it has been suggested that additives be added to the electrolyte: For example, Japanese Patent Laid-open No. 95-176323 discloses an electrolyte that has added CO2, and Japanese Patent Laid-open No. 95-320779 discloses an electrolyte with a sulfide-based compound additive in order to prevent the electrolyte from being decomposed.